P HOTORESPIRATION Let us try and understand one more process that creates an important difference between C and C plants – Photorespiration . To understand photorespiration we have to know a little bit more about the first step of the Calvin pathway – the first CO fixation step. This is the reaction where RuBP combines with CO to form molecules of 3PGA, that is catalysed by RuBisCO. RuBP CO PGA RuBisCo → × RuBisCO that is the most abundant enzyme in the world (Do you wonder why?) is characterised by the fact that its active site can bind to both CO and O – hence the name.
Can you think how this could be possible? RuBisCO has a much greater affinity for CO when the CO : O is nearly equal. Imagine what would happen if this were not so! This binding is competitive.
It is the relative concentration of O and CO that determines which of the two will bind to the enzyme. In C plants some O does bind to RuBisCO, and hence CO fixation is decreased. Here the RuBP instead of being converted to molecules of PGA binds with O to form one molecule of phosphoglycerate and phosphoglycolate ( Carbon) in a pathway called photorespiration. In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP.
Rather it results in the release of CO with the utilisation of ATP. In the photorespiratory pathway there is no synthesis of ATP or NADPH. The biological function of photorespiration is not known yet. In C plants photorespiration does not occur.
This is because they have a mechanism that increases the concentration of CO at the enzyme site. This takes place when the C acid from the mesophyll is broken down in the bundle sheath cells to release CO – this results in increasing the intracellular concentration of CO . In turn, this ensures that the RuBisCO functions as a carboxylase minimising the